1. Technical Field of the Invention
The present invention relates to mobile communications and, more particularly, to a method and a system for handling a mobile terminating call in a mobile communications network, comprising a plurality of Mobile Switching Centres (MLCs).
2. Description of Related Art
In a mobile telecommunications network, such as the Global System for Mobile communications (GSM), a user may roam nationally and even internationally. This has consequences for the architecture of such networks and the manner that calls are routed to mobile terminals. Routing starts as soon as a calling telecommunications subscriber dials the directory number of a called mobile subscriber. The number dialed to reach a mobile subscriber is called the Mobile Subscriber Integrated Services Digital Network (MSISDN). This number includes a Country Code and a National Destination Code, which identifies the subscriber's operator. Furthermore, it includes a subscriber number identifying the subscriber. The MSISDN may also identify the subscriber's Home Location Register (HLR) within the Home Public Land Mobile Network (HPLMN). An HLR is a database that contains information about subscribers to a mobile network. The HLR registers subscribers for a particular service provider. Rather than temporary subscriber data, which is managed by a Visited Location Register (VLR), the HLR stores “permanent” subscriber information including the service profile, and a VLR address associated to the subscriber.
An incoming request for setting up a call to a mobile terminal, in other words setting up a Mobile Terminating (MT) call, is directed to a Gateway Mobile Switching Centre (GMSC) function. The GMSC receiving a request for setting up an MT call determines the Mobile Switching Centre (MSC) that is currently able to reach the called mobile subscriber—in other words, which MSC service area it must connect to. It shall be noted that the GMSC function is distinct from the MSC function, but is usually implemented in an MSC.
For the purpose of determining the appropriate MSC, the GMSC sends a message called Send Routing Information (SRI), which is a Mobile Application Part (MAP) message, to the HLR to request routing information for routing the MT call towards a mobile subscriber. MAP is a protocol, which enables real time communication between nodes in a mobile cellular network. The MAP protocol is also used for the transfer of location information from the VLR to the HLR. In order for the GMSC to be able to interrogate the subscriber's HLR to obtain routing information in this way, the GMSC may address a Signal Transfer Point (STP), which is an element of an SS7-based Intelligent Network that performs routing of the SS7 signalling, and which contains a table linking MSISDNs to their associated HLR.
The HLR informs the GMSC of which MSC to use, and because the called mobile does not have a fixed extension to that particular MSC, the HLR will also provide the GMSC with a temporary routing number, the so-called Mobile Station Roaming Number (MSRN), to be used for signalling. MSRNs are related to the geographical numbering plan, and not assigned to subscribers, nor are they visible to subscribers. After the call is routed to the particular MSC, the International Mobile Subscriber Identity (IMSI) code corresponding to the MSRN is looked up by the particular MSC, and the mobile is paged in its current location area.
In relation to a request for setting up a call, also services may be provided. One way to provide services is by means of an Intelligent Network (IN), which is a modern communications technology, representing an extension of traditional telecommunications architecture, especially that of analog and ISDN networks. IN allows swift and easy introduction of new services.
For roaming subscribers the Customized Applications for Mobile network Enhanced Logic (CAMEL) allows subscribers access to their full portfolio of IN services. CAMEL connects the home and visited mobile networks to various IN platforms used throughout national networks to provide features such as Pre-Paid Calling, Personal Numbering and more complex location dependent services. As a result, CAMEL is a relatively inexpensive method of allowing telecom operators to add new services in to the existing network infrastructure.
CAMEL standard is part of GSM. The main object of CAMEL is to support operator specific services, which are not covered by standardized GSM services to a mobile subscriber, even when the mobile subscriber is roaming outside the HPLMN, i.e. to specify therefore required inter-network interfaces and underlying networks. The CAMEL feature is not a supplementary service, but a network feature to help the network operator. It is, however, also possible to deploy CAMEL for intra-network usage and to use the part of the CAMEL feature being supported by HLR, MSCNLR (Mobile services Switching Centre/Visitor Location Register) and GMSC for invocation of IN based services, which are triggered in a SSF (Service Switching Function) being integrated with the MSC or GMSC, and for routing to stand-alone SSPs (Service Switching Point) and service nodes in the mobile telecommunications system.
For particular groups of CAMEL services, announcements to be played to a subscriber are subscriber-specific. An example of such a CAMEL service is a Personal Greeting Service (PGS). A PGS service may also be based on other protocols such as CS1+. A PGS service plays a customised announcement during call setup. The announcement to be played may (typically) depend on the served subscriber, the calling party or the called party.
WO 98/53626 discloses a method and an arrangement of functional entities in a mobile telecommunications system for supporting, to a mobile subscriber, a second set of operator specific services, having a high level of functionality, additional to services standardised in the system. A first set of operator specific services, with a lower level of functionality, is supposed to already be provided for in the system. The second set of services is provided for by a telecommunications standard different from the standard which provides the first set of services, and is preferably a structured standard for mobile telecommunication. The second set of services is triggered by an IN capability indicator, that is included in a subscription information field stored for each subscriber in a subscriber database.
One such example of a second set of services is specified by CAMEL, which specifies Terminating CAMEL Subscription Information (T-CSI) as the IN trigger information for MT call handling.
If an operator wishes to invoke IN services that use capability that goes beyond the CAMEL capability set, then that operator has to use proprietary IN trigger information elements, hereinafter referred to as IN Trigger Elements. As an example, when the HLR sends an IN Trigger Element to the GMSC, then the GMSC may invoke a vendor specific IN service for the MT call. Using an IN Trigger Element for MT call handling has the requirement that both the GMSC and the HLR are from a single vendor.
Examples of IN services for MT calls, which may be vendor-specific services include                Personal Greeting Service        Virtual Private Network.        
A special, though not uncommon situation appears when a mobile communications network comprises both GSM and TDMA access technologies. TDMA (Time Division Multiplex Access) comprises a type of multiplexing where two or more channels of information are transmitted over the same link by allocating a different time interval (“slot” or “slice”) for the transmission of each channel; that is, the channels take turns to use the link. Some kind of periodic synchronizing signal or distinguishing identifier usually is required so that the receiver can tell which channel is which.
FIG. 3 shows a graphical representation of such a combined GSM & TDMA network. When a call for a subscriber of a combined network arrives, the call needs to be handled by the GSM network or by the TDMA network, depending on the network the called subscriber belongs to. However, the MSISDN (i.e. the called party number) does not indicate which network the call shall be sent to, as explained earlier.
Practically, the following prior art solutions may be used:                the HLRs in one of the two networks, e.g. the TDMA network, contain the full MSISDN range, encompassing all GSM subscribers and all TDMA subscribers. All calls are routed to that network. When the subscriber belongs to the other network, then the HLR returns a routing number. The GMSC then uses the routing number to route the call to the other network. In the other network, HLR interrogation takes place again;        all calls are routed to one of the two networks. If the HLR in that network does not hold a record of that number, it rejects the call and the call is then routed to the other network.        
The above described prior art solutions have the following disadvantages:                when an operator has a network with GMSCs from different vendors, then it is not possible to ensure that for selected individual subscribers, the MT call handling is always done by particular GMSCs in that network. MT call handling by a particular GMSC may be needed, amongst others, if vendor-specific IN handling from that GMSC is required;        additional signalling and additional HLR provisioning are required. In practice, when e.g. a TDMA operator wants to offer GSM as well, that operator would prefer to leave the TDMA network unaffected, but adds just the GSM network. That means, amongst others, that the HLRs in the TDMA network should have no knowledge about the GSM subscribers. These operators typically use a single MSISDN range, with each subscriber being a GSM subscriber or a TDMA subscriber. Hence, the MSISDN of a subscriber of that network does not indicate whether that subscriber is a GSM subscriber or a TDMA subscriber. Using dedicated MSISDN ranges for GSM and TDMA subscribers is not desirable. Such approach would hamper migration between TDMA and GSM (or CDMA and GSM etc.);        if a vendor specific service needs to be used in a heterogeneous network, with HLRs from multiple vendors and GMSCs from multiple vendors, there is the following dilemma:        the vendor specific IN Trigger Element may not be administered in HLRs which are not of the same vendor (non-vendor HLRs);        the vendor specific IN Trigger Element may not be used in GMSCs, which are not of the same vendor (non-vendor GMSC).        
As a result, for handling of MT calls it may be desirable for a network to have selected IN subscribers located in a dedicated HLR and to have HLR interrogations for these selected subscribers done by one or more dedicated GMSCs, able to provide an IN service for the call to the subscriber.
However, for a current art GSM network, further disadvantages are:                any of the MSCs in the network may function as GMSC, so HLR interrogations for a subscriber to an operator specific service may come from both dedicated GMSCs and non dedicated GMSCs;        a subscriber who wishes to subscribe to an operator specific service, requiring a vendor specific IN Trigger Element, may happen to be located in a non-vendor HLR. An operator cannot easily move subscribers between HLRs, since subscribers' IMSI and MSISDN are tied to the address of the HLR where the subscription data of those subscribers is located.        